The present invention relates to Class I and Class II power transformers. More particularly, this invention relates to a universal power transformer that includes an internal design to produce one or more additional sets of living bushing locations than on a standard designed transformer. The standard design of a transformer includes two sets of bushings, one set of bushings for high voltage and one set of bushings for low voltage. The transformer is than coupled with a means for easy attachment and detachment of the bushings to and from the power transformer. The present invention further includes a means for capping unused bushing extensions in a power transformer, thus allowing the power transformer to be used in various substation designs which require different transformer arrangements and voltages. Accordingly, the present invention provides a coupling means for a power transformer such that the power transformer has the flexibility heretofore associated with distribution type transformers.
A grid of power lines typically distributes electricity at various voltages. These power lines originate from a source; a main generator station. From the generating station, the lines carry the power to numerous substations on a power system. The voltage of this electricity at the generating station is stepped up for efficient conducting of electric power. Each substation contains a power transformer that is used to step down the voltage to lower levels for subsequent conducting through a distribution network. The distribution network includes numerous smaller distribution transformers, which lower the voltage to levels conveyed to commercial buildings, households, and other end users. Conventionally, the input voltage is called the primary voltage and the output voltage is called the secondary voltage.
Thus, there are two types of transformers used to convey electricity from a transmission system to a distribution system. These transformers are power transformers and distribution transformers.
Power transformers, which are used in the substations, must process far greater influxes of electric current and voltages than distribution type transformers. Each substation can have a unique layout depending on available space and type of equipment connected to the power transformer. The equipment connected could include switchgear, open bus conductor, etc. As such, power transformers are normally designed to conform to unique substation requirements. Thus, it is sometimes difficult to utilize an inventoried power transformers as replacements. This difficulty occurs even within a single utility's electric system, as the power transformer available may not be of the necessary design configuration.
On the other hand, distribution type transformers are designed for much lower levels of electric power. Distribution transformers are typically much smaller in size and are not usually constrained by the installation site. In fact, most distribution transformers are mounted on utility poles, on ground level pads or placed under ground. The lack of design constraints for distribution transformers has resulted in a standardized design that is readily substitutable.
Heretofore, the prior art has failed to provide a similar flexibility for the power transformer class. Thus, there is a need for a new and novel universal power transformer. Such a novel universal transformer would reduce potential outage time as suitable replacements would be more readily available. The novel transformer would reduce the cost associated with new substations, as well as the transformer itself, since the multiple bushing location requirements of the user could be identified and standardized. The novel transformer with multiple bushing locations coupled with multi-rates primary or secondary voltages would even further extend the flexibility of the universal power transformer. The novel transformer would reduce the number of spare transformers needed to ensure system reliability, and would permit relocation of power transformers from one substation to another, or from one planned design to another. Such a new universal transformer would likely have the ability to connect properly and safely to the cables or ridged bus carrying the power to and from the electric system.
Since each substation can have a unique layout, the novel universal transformer must be able to connect to an electrical power cable or a ridged type bus located in a variety of configurations. When a power transformer is specified for an individual substation, the particular power transformer is built to address the arrangement, ratings, and design concerns for that installation. Thus, for the power transformer to be useful in more than a single installation, the power transformer must be capable of accepting cables or ridged type bus connections at multiple locations located on the transformer exterior.
Accordingly, the novel invention provides multiple sets of bushing wells seated in apertures in a power transformer tank to provide the desired flexibility. Once a transformer arrangement is chosen and the installation is made to a new or existing substation, the remaining open bushing wells are capped to protect them from contamination, to reduce the likelihood of tracking, to insulate the live terminal from the environment, to prevent accidental contact with the live terminal at the base of the well, and to prevent corona activity from the live terminal. This cap becomes a dead front cover to the well. Installation of the cap reduces the potential for contamination from dirt, weather, birds and small animals. The unused bushing wells are covered for purposes of safety as well as for aesthetics.
The novel invention discloses live connections in multiple segments of the power transformer which require no switching devices. Thus, within the power transformer of this invention, multiple sets of permanent high voltage or low voltage connections are made depending on the users requirements. Unused wells are effectively capped or insulated from the environment in accordance with the disclosure of the invention. No method of switching or switching device is required between the power transformer's permanent connections.
The novel invention discloses a bushing disconnect as a means to facilitate easy relocation of bushings from one segment or location to another segment or location. The removal of the bushing disconnect from the well does not require access into the power transformer itself as occurs with the removal of a standard type oil immersed transformer bushing. Attachment of the bushing disconnect to the bushing well is accomplished with a simple type of connecting device. The bushing disconnect is air insulated on both the inside and outside of the bushing well.
The novel invention discloses that the removal or the relocation of a bushing disconnect from one segment of a power transformer to another segment of the transformer, could take as little as fifteen (15) minutes. The removal or relocation of a standard bushing in a standard type power transformer could take anywhere from several hours to two (2) to four (4) days to accomplish the required task. Removal or relocation of the bushing disconnect in fifteen (15) minutes as compared to two (2) to four (4) days for a standard bushing results in a substantial cost savings. The novel bushing disconnect also substantially reduces outage time which occurs as a result of the removal or relocation of a bushing, eliminates possible oil spills, and reduces the risk that contaminants enter the power transformer.
The novel invention for the universal power transformer discloses a unique air gap between the bushing well and the bushing disconnect of a minimum volume sufficient to accommodate the incident of thermal expansion and to prevent corona activity. The distribution type bushing insert assemblies typically found on pad mount power transformers, used with underground cable systems, require that the assembly expel all the air between connecting parts to prevent corona activity. Often times a gap-filling lubricant is used to ensure a tight fit between the bushing and the bushing well.
U.S. Pat. No. 4,360,849 issued to Harris, et al. in 1982 discloses a complete substation modular package. The system as disclosed includes a transformer, primary circuit breakers, low voltage switchgear, and accessories required for the operation of the substation module. Harris teaches a design of a transformer where the high voltage bushing and low voltage bushing are in a fixed position. The Harris transformer design is adaptable to a single configuration. The power transformer described in the Harris design shows the low voltage bushings located in segment 1 on the side and the high voltage bushings in segment 3 on top, a standard design in the art for power transformer arrangements. These bushings are in a fixed location and this power transformer has no flexibility to adapt to other substation arrangements if needed. In addition, to remove a standard bushing in the low voltage position requires that the power transformer oil be drained to some point below the bottom connections of the bushings; the connections being internal to the transformer. After the oil is drained, the user enters the power transformer through port holes or manholes located on the transformer in order to gain access to the leads which are attached to the bottom of the bushings. The removal of these standard type bushings on the Harris design could reasonably take up to four (4) days to complete, including the oil processing, testing and replacement of the bushing.
U.S. Pat. No. 4,863,392 issued to Borgstrom, et al. in 1989 and assigned to Amerace, Inc. of Parsippany, N.J., refers to an improvement to a loadbreak bushing insert for use on distribution underground cable systems and equipment such as pad mount power transformers. This design may be used in conjunction with a power cable elbow and bushing insert as evidenced in a product provided by Amerace, Inc. and shown in a catalog entitled "15 kV Loadbreak Connectors Power Cable Elbow Bushing Insert," having the catalog number C-410M2 and issued on March 1989 (hereinafter referred to the "Amerace Design"). The Amerace Design is for underground distribution equipment connections. The bushing inserts as described in the Amerace Design, mate with the power cable elbow only for connection to underground cable systems and nothing else. The Amerace Design is for enclosed use only and is a dead front design. The dead front design refers to an ability to safely touch the equipment while the equipment is energized.
The Amerace Design bushing inserts are screwed onto the bushing well terminal or stud as depicted in U.S. Pat. No. 4,353,611, issued to Siebens, et al. in 1982. The Siebens' patent discloses an improvement to bushing well construction. Siebens teaches the use of a replaceable threaded stud, but does not provide for an improved connection between the bushing well and the bushing. The Siebens' patent discusses how the studs in prior art designs apparently snapped or were rather easily damaged. The bushing as disclosed in Siebens, is intended to work with underground distribution cables. The material as disclosed in Siebens cannot be used in conjunction with Class I or Class II power transformers.
The bushing design of the present invention can be distinguished from the bushing design as described in patent '611. The present invention is directed to an outdoor power bushing with outdoor air clearances and creep design. The present invention is not a dead front design, is not used in conjunction with underground cable installations, and does not break load for switching applications. The bushing design of the present invention incorporates a tulip type connection not threaded on the bottom and when installed connects to a stud molded into the bushing well specifically designed for this application with air clearances. On the top of the novel bushing, as disclosed herein, is a standard threaded conductor to connect to an outdoor bus work or bus duct via a stud connector. The novel bushing is designed to create a universal power transformer with a user friendly means of changing the bushings from one location on the power transformer to another location on the power transformer, or on a standard type power transformer to make bushing replacement simple and environmentally safe.